Japanese astronomers say they have found traces of a mineral that adds an important piece of knowledge to the puzzle of the Moon's geological past.

Using sensors onboard the Kaguya spacecraft, which was placed in orbit around the Moon in 2007, the team found abundant signatures of the mineral in concentric rings in three big crater regions.

The mineral, called olivine, is deemed to be a telltale of mantle, the deep inner layer of iron- and magnesium-rich rock that lies beneath the Moon's crust.

Headed by Satoru Yamamoto of the National Institute for Environmental Studies in Tsukuba, the study appears online in the journal Nature Geoscience.

A leading theory is that the Moon was created about 4.5 billion years ago after our planet suffered a gigantic collision from a Mars-sized object, forming two objects - the Earth and the Moon.

As the material coalesced into a ball, its surface gradually cooled, forming a crust made of a light-coloured aluminous mineral, feldspar, which floated in a dense, molten liquid.

Kaguya's data add a chapter to this "lunar magma ocean" hypothesis.

Thick crust

It suggests that after the crust had formed, there was some massive overturn in the fiery liquid beneath. Olivine-rich mantle was brought from deep within the lunar bowels to within the base of the crust.

At the craters sampled by the probe - the South Pole-Aitken, Imbrium and Moscoviense impact basins - the Moon's crust is very thin, and the olivine mantle may have been exposed by asteroids that smashed into the lunar surface, the paper suggests.

The Moon's crust is believed to be far thicker than Earth's, averaging around 70 kilometres in depth, although the thickness varies significantly.

The structure and origins of the Moon's mantle have been fiercely debated by astro-geologists.

The famous Moon rocks brought back by the Apollo missions shed no light on the question, as they were all from the lunar crust. As a result, pointers to the existence of the mantle were, for decades, either sketchy or indirect.

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